Numerous spinal vertebrae fractures occur each year, many in older women as a result of osteoporosis. The pain and loss of movement accompanying vertebral fractures severely limits activity and reduces the quality of life. In contrast to typical bone fractures, the use of surgery to treat vertebral fractures is extremely difficult and risky. A procedure called “vertebroplasty” is a less-invasive alternative to surgery, with fewer attendant risks, and has proved extremely effective in reducing or eliminating the pain caused by spinal fractures.
Vertebroplasty involves injecting radiopaque bone cement into the damaged vertebral body by way of a needle or cannula using x-ray (fluoroscopy) to visualize and monitor delivery. Generally, vertebroplasty is performed by radiologists, neurosurgeons, and orthopedic surgeons.
Directly prior to injection, bone cement is prepared by mixing bone-cement powder (e.g., polymethylmethacrylate “PMMA”), liquid monomer (e.g., methyl methacrylate monomer), with an x-ray contrast agent (e.g., barium sulfate), to form a fluid mixture. The components of bone cement must be kept separate from each other until the user is ready to mix them to form the desired bone cement. Typically, bone-cement powder is stored in a flexible bag, pouch, bottle, or similar container, while the liquid monomer is stored for shipment and handling in a vial or tube, usually formed from glass. Bone cement sets and hardens rapidly, so the doctors must work quickly and efficiently. A typical bone-cement mixture may comprise 15 g polymethylmethacrylate powder, 5–10 g of methyl methacrylate monomer, and 5–8 grams of sterile barium sulfate for radiographic visualization of the cement. The radiopaque bone-cement mixture is placed in a cannula-type dispensation system, the needle portion is inserted into the patient, properly positioned, and the bone cement slowly injected into the subject vertebra using x-ray guidance allowing the doctors to see the mixture actively infuse. When enough of the cement is injected into the damaged bone, as seen by x-ray, the flow is stopped and the needle is removed. However, as discussed below, stopping the flow is easier said than done. There are serious control problems with current cannula-type bone-cement dispensation systems.
While the procedure itself has proven very effective, problems are associated with handling and mixing the bone cement. Bone cement hardens very quickly, even more so upon exposure to air. Also, it is important that the cement delivered into the bone be virtually free of any entrapped air bubbles or air pockets. In spite of this, bone cement is typically hand mixed in an open environment directly before the procedure using a tongue depressor or spatula. The mixed cement is then manually transferred from the mixing vessel to a separate dispensing device, such as a syringe. Removal of the mixed cement from the mixing vessel into the caulking gun or syringe is cumbersome, time consuming, and has the potential for being mishandled, dropped or contaminated. In any case, the resulting bone cement, since it has been exposed to air, is less fluid and harder to force through the cannula into the vertebrae. Accordingly, more pressure must be exerted by the attending physician on the dispensing device. The increased pressure requirement makes control difficult and increases the likelihood that too much cement will be injected. For example, when the x-ray indicates that the vertebrae is filled, it is difficult to stop the cement flow out of the cannula and overflow of the cement into the surrounding tissues can result. This is unsafe for the patient since the excess cement may leak out of the vertebral body into surrounding tissue and vascular structures. In some cases, surgery may be required to remove the excess cement.
Another disadvantage with current bone cement mixing protocols that require open-air transfers stems from the toxic nature of the liquid monomer component. Bone cement monomers, including methyl methacrylate, give off toxic vapor and are irritating to the eyes and respiratory system. Furthermore, acrylate monomer irritates skin and contact with minute concentrations can cause sensitization. Accordingly, handling requires the use of suitable gloves. So, not only must attending clinicians worry about the deleterious effects of incorporating air bubbles into the bone cement during the cumbersome hand mixing, but also be concerned with health and safety issues in connection with toxic methyl methacrylate vapors.
Currently, many clinicians begin the bone-cement mixing process by first opening a glass vial containing the liquid monomer component. One common method for opening glass vials is to snap off the top of the vial at the smallest cross section. Unfortunately, this method risks injury to operating-room personnel from broken glass or sharp edges. Another disadvantage is that small glass shards often form during such breaking, which can fall into the cement mixture. In attempting to expedite the opening of the vial or tube holding the liquid monomer, as well as reduce any exposure to the foul odor possessed by the liquid monomer, various prior art systems have been developed for enabling the user to insert the sealed vial or tube into an area of the vessel and then break the vial or tube for releasing the liquid monomer directly into the dry powder.
These prior art systems all require that the broken glass pieces or shards of the vial/tube must be separately retained and prevented from reaching the bone cement product. In attempting to satisfy this requirement, substantial construction and operational difficulties have occurred with these prior art systems. Furthermore, in other prior art systems, manual addition of the monomer is required, exposing the user to the foul odor of the monomer and the substantial difficulties typically encountered in handling such products.
What is needed is a mixing and dispensing device that can mix the components of bone cement in a sealed environment and provide increased control on dispensation so that the operator can readily stop the bone-cement flow when the desired amount has been dispensed.